1. Field of the Invention
The present invention relates to a radio communication system, a radio communication method and a base station, in which a base station transmits, to a radio terminal, transmission rate control data for controlling a transmission rate of uplink user data.
2. Description of the Related Art
Heretofore, a radio communication system including a base station and a radio network controller has been known. In the radio communication system, the base station has a single or multiple cells, and a radio communication is performed between each of the cells and a plurality of radio terminals. The radio network controller controls a plurality of base stations, and assigns a radio resource to the plurality of the radio terminals. Note that such a technique (hereinafter referred to as first technique) is sometimes referred to as R99 (Release 99) or the like.
For the purpose of improving the throughput and shortening the delay time, and the like, there has recently been proposed a technique in which a base station assigns the radio resource to uplink user data transmitted from each of the radio terminals to the base station (network side). Note that such a technique (hereinafter referred to as a second technique) is sometimes referred to as the high speed uplink packet access (HSUPA), the enhanced uplink (EUL) or the like.
In the above-described second technique, each of the radio terminals transmit uplink user data to the base station via an enhanced dedicated physical data channel (E-DPDCH). A transport block size (TBS) is determined based on the transmission rate (for example, a scheduling grant (SG)) of the uplink user data, and is controlled by an absolute grant (AG) and a relative grant (RG), which are transmitted from the base station to each of the radio terminals.
Here, the radio terminals can transmit a happy bit to the base station. The happy bit indicates whether the SG controlled by the AG or the RG is sufficient for each of the radio terminals. The happy bit includes “Happy” indicating that the SG is sufficient for the radio terminals, and “Unhappy” indicating that the SG is not sufficient for the radio terminals (for example, see 3GPP TS25.321 Ver. 7 11.8.1.5 “Happy Bit Setting”).
When a happy bit rate, which indicates a ratio of “Happy” to all of the transmitted happy bit (hereinafter referred to as “Happy Bit Rate”) is lower than a predetermined threshold, the base station increases the SG to be assigned to the radio terminal.